1. Field of the Invention
The present invention relates to a thermosetting coating composition in a powdery form, and a coating film prepared by curing the above coating composition. More specifically, the present invention relatives to a thermosetting coating composition in a powdery form comprising, as the main components, two or more resinous components chemically bonded to each other or suitably dispersed therein, and a coating film prepared by curing the above coating composition.
2. Discussion of the Related Art
The resin materials usable in the preparation of powder coating may be roughly classified to thermosetting resins and thermoplastic resins. When a thermoplastic powder coating is applied, heat is supplied to a powder coating on the base material (undercoating), to thereby melt particles of the powder coating, so that the particles are made flowable to form a levelled coating film.
On the other hand, when compared to coating films prepared from a thermoplastic resin, a coating film prepared from a thermosetting resin is generally more tough and more resistant to solvents and detergents, and shows better adhesion to metal base materials, so that the coating film prepared from a thermosetting resin does not undergo softening when exposed at a high temperature. However, aside from the desired properties mentioned above, the curing of the thermosetting resin coating film gives rise to the following problems in order to obtain a coating film having good levelling property and flexibility.
In other words, the coating films prepared from the thermosetting resin powders are likely to be cured or solidified upon heat application before the formation of a levelled coating film, thereby giving a relatively rough finish which is often referred to as "orange peel" surface. In such a case, the coating film substantially has no gloss or luster on the orange peel surface. In order to solve the problem of the orange peel surface, proposals have been made to use thermosetting resin coatings using an organic solvent as a dilution agent. However, in these thermosetting resin coatings, the organic solvents are evaporated and released to the air, which gives rise to undesirable environmental and safety problems.
Besides having good gloss, high impact strength, and high resistance to solvents and chemicals, the coating films prepared from the thermosetting resin coatings have to have excellent flexibility. For instance, metallic sheets are designed to be molded by deflection or bending at various angles in the manufacturing of automobiles, etc., and it is essential to have good flexibility in the thermosetting resin coating film.
In order to meet such demands, U.S. Pat. No. 4,352,924, of which the disclosure is incorporated herein by reference, discloses a particular crystalline polyester useful for the powder coating composition. Also, WO 89/65320, of which the disclosure is incorporated herein by reference, discloses a powder coating composition comprising an amorphous polyester, a semi-crystalline polyester, and a polyisocyanate crosslinking agent.
However, coating films prepared from these powder coating compositions disclosed above do not have a sufficient level of weatherability.
An object of the present invention is to provide a thermosetting coating composition which gives excellent gloss, levelling property, chroma, and weatherability in the resulting coating film prepared therefrom, and also gives an excellent storage stability in the resulting coating.
Another object of the present invention is to provide a coating film prepared from the above thermosetting coating composition.
These and other objects of the present invention will be apparent from the following description.